A DESCRIPTION OF THE PROBLEM :
The width of the ComboBox is changed when a value is selected.
<LINK><ATTACHED GIF>

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run the provided code. Stretch the stage width to increase the window width by approximately one and a half times. Select a value in the ComboBox.
Code:
public class NewMain extends Application {

     enum Test { FOO, BAR }

     @Override
     public void start(Stage stage) {
         GridPane gridPane = new GridPane();
         gridPane.setHgap(10);
         gridPane.setVgap(10);
         gridPane.setPadding(new Insets(10));

         textFieldRow(gridPane, 0);
         textFieldRow(gridPane, 1);

         Label label1 = new Label("Label AAAAA BBBBB:");
         label1.setMinWidth(Region.USE_PREF_SIZE);
         var comboBox = new ComboBox<Test>(); comboBox.setItems(FXCollections.observableArrayList(Test.values()));

         gridPane.add(label1, 0, 2);
         gridPane.add(comboBox, 1, 2);
         comboBox.setMaxWidth(Double.MAX_VALUE);

         Scene scene = new Scene(gridPane, 600, 200);
         stage.setScene(scene);
         stage.show();
     }

     private void textFieldRow(GridPane gridPane, int index) {
         Label label1 = new Label("Label AAAAA BBBBB:");
         label1.setMinWidth(Region.USE_PREF_SIZE);
         TextField textField1 = new TextField();
         GridPane.setHgrow(textField1, Priority.ALWAYS);
         textField1.setMinWidth(10);
         Label label2 = new Label("Label CCCCCC DDDDDD:");
         label2.setMinWidth(Region.USE_PREF_SIZE);
         TextField textField2 = new TextField();
         GridPane.setHgrow(textField2, Priority.ALWAYS);
         textField2.setMinWidth(10);

         gridPane.add(label1, 0, index);
         gridPane.add(textField1, 1, index);
         gridPane.add(label2, 2, index);
         gridPane.add(textField2, 3, index);
     }


     public static void main(String[] args) {
         launch(args);
     }
}